An exhibition in Paris is shedding light on the previously unknown influence of Claude Monet's older brother, Leon, in the life and art of the French Impressionist painter. Leon, who was a color chemist and four years older than Claude, is now recognized as having played a crucial role in his brother's commercial success and the development of the famous color palette that produced masterpieces such as the "Water Lilies" series.

"It’s never been known before, but without Leon there would not have really been a Monet - the artist the world knows today," said Geraldine Lefebvre, exhibit curator at the Musee du Luxembourg.

"His rich big brother supported him in the first period of his life when he had no money or clients and was starving," she said. "But more than that. The vivid palette Monet was famous for came from the synthetic textile dye colors Leon created" in the town of Rouen - the site of some of Claude's best-known paintings.

The groundbreaking exhibit is the fruit of years of investigation by Lefebvre, who visited Monet’s great-grandchildren, studied family albums and brought to light a masterly portrait of Leon by Claude that Leon hid away in a dusty private collection and has never before been seen by the public. The 1874 painting shows Leon with a black suit, stern expression and red – almost wine-flushed – cheeks.

The exhibit dispels a long-held view that Claude and his older brother were estranged.

"Historians always thought the two brothers had nothing to do with each other. It was assumed that there are no photographs of Claude and Leon together and no correspondence. In reality, they were incredibly close throughout their life," Lefebvre said.

The brothers had an argument in the early 1900s and that may explain why no direct traces of the relationship exist. "Maybe Leon got rid of the traces, maybe it was Claude. Maybe it was jealousy. We will never know. It is a mystery," Lefebvre said.

What is now known is that Leon would dine with his younger brother, introduce him to other artists, give him money, and patronize his art – buying it up at auction at high prices to boost his reputation.

"One of the problems was because they shared the surname it seemed like (Claude) Monet was buying back his own pictures. But it was Leon," said professor Frances Fowle, senior curator of French art at the National Galleries of Scotland.

"This exhibit is important as it throws light on Leon Monet, who up until now has been an invisible figure. It also reveals the wider network at work. Leon was a key figure," Fowle added.

Leon’s influence went beyond his brother: He financially supported other impressionists such as Camille Pissarro, Auguste Renoir and Alfred Sisley – some of whom would connect around his dinner table in Rouen, where the wine would flow freely. Claude followed his brother to Rouen, where he painted his Rouen cathedral masterpieces.

Monet also worked for his older brother as a color assistant, a pivotal moment not only in his life – but possibly in the emergence of impressionism as we know it.

Leon would dissolve carbon to create a chemical called aniline, which created incredible synthetic colors that natural pigments couldn't compete with. One of the earlier examples of Leon’s color filtering down into Monet’s art is from an 1860s illustration – before he was famous – that is featured in the exhibit. Monet drew his future wife Camille in a dress with an eye-popping green that had never been seen before.

"The French press coined the term ‘Monet green,’" Lefebvre said, adding that journalists were initially mocking it. "At the time, they said he would make a good dye artist."

However, both Monets had the last laugh.

Claude Monet founded impressionism – a term coined from his 1872 painting "Impression, Sunrise" – to become one of the most celebrated painters of the last two centuries. And by impressionism's height at the end of the 19th century, an incredible "80% of all impressionists’ work" used the synthetic colors borrowed from Leon, according to Lefebvre.

These synthetic hues, which were cutting-edge at the time, enabled members of the group to depict the fleeting impression of the moment with shifting colors and luminosity.

"Who knows the exact extent of the impact Leon had on the movement?" Lefebvre said with a shy smile. "But it was extraordinary."

"Leon Monet. Brother of the artist and collector" runs at the Musee du Luxembourg in Paris from March 15 until July 16.